The present invention relates generally to a homogeneous field magnet having a yoke which conducts the magnetic flux and two opposing pole shoes between which there is formed a useful volume having a magnetic field of high homogeneity, each pole shoe being provided with a pole piece means which is spaced from a base part facing the yoke of the corresponding pole shoe by a narrow correction air gap and which contains at least two pole piece elements of different material which are parallel to each other. EP 0 488 015 A1 discloses such a homogeneous field magnet.
Homogeneous field magnets are necessary, in particular, for the production of magnetic base fields in systems for nuclear-magnetic-resonance tomography, imaging or spectroscopy. For this purpose, the magnetic field of such base field magnets must be sufficiently homogeneous in an imaging or exploration region (useful volume) and produce therein a predetermined magnetic induction B.sub.0. In general to serve this purpose, superconductive coil systems are provided for magnetic inductions B.sub.0 of more than 0.5 T. As compared with this, smaller magnetic inductions, B.sub.0 &lt;0.5 T, can also be produced with normally conducting coils or permanent magnets. These last-mentioned magnets are frequently developed as so-called pole shoe magnets having a magnet yoke in the form of a "C" or "H". Between the pole faces of opposing pole shoes, there is then present the useful volume with the required field homogeneity. In particular, for the requirements of nuclear magnetic resonance tomography, the initially obtainable field homogeneity in the useful volume is insufficient due to unavoidable manufacturing tolerances. Rather, it must be possible to correct for these manufacturing tolerances in a finished magnet in order to be able successively to reduce the field error by an alternating sequence of field measurements and field corrections (i.e., the so-called "shim procedure").
The aforementioned EP-A discloses a pole shoe magnet in which the field is correctable via mechanical means. For this purpose, its pole shoes are formed of adjustable pole piece means in the region of its pole faces. Furthermore, the surfaces of the pole shoes which face the useful volume can be so profiled to compensate for edge effects which affect homogeneity.
In order to be able to align such pole piece means with respect to each other with sufficient accuracy and to correct field errors, none of the pole piece means in the known homogeneous field magnet are fastened directly to the magnetic yoke conducting a magnetic flux. Rather, each pole piece means is spaced from a base part of the pole shoe which faces the yoke by a narrow correction air gap, and the means are made suitable and/or flexible by special setting means. In this arrangement, the air gap acts as magnetic series resistance to homogenize inhomogeneities of flux density in flux conducting parts of the pole shoe upon passage into the corresponding pole piece means.
In accordance with one particular embodiment of the known homogeneous field magnet, each of its pole piece means can be formed by a stack-like construction consisting of two layer-like pole piece elements of approximately the same thickness, different soft-magnetic materials being used for these elements. The first pole piece element, which faces the correction air gap, can be formed, for instance, of at least one sheet of electro-engineering consisting of an Fe-Si alloy, whereby sufficient flexibility of the element is to be assured. As compared with this, the second pole piece element, which faces the useful volume, can consist for instance of a soft-magnetic ferrite or a resin bonded iron powder. With a relative permeability .mu..sub.r (=.mu./.mu..sub.0 =1/.mu..sub.0 * B/H) of about 1000 and a flux-carrying capacity B.sub.max of about 0.4 T to 0.5 T of the material of this second layer, this material advantageously has only a slight electrical conductivity. On the other hand, the first layer is to provide for good base-field homogeneity. For this purpose, it consists of a material having a relative permeability .mu..sub.r of between 1000 and 5000, with, at the same time, a high flux-carrying capacity B.sub.max of about 1.6 T.
Such pole-piece means also bear a magnetic field produced by pulsed gradient coils. It is then found, however, that a magnetic hysteresis of the pole-piece material leads to residual fields after a gradient pulse and to a non-linear current-field relationship. As a result, the quality of the image of known imaging processes, such as, for instance, the so-called turbo-spin-echo method, are impaired. The occurrence of hysteresis phenomena is also a cause of errors in the magnetic base field. Depending on whether an operating current is obtained from existing exciter coils of lower or higher values, a different spectrum of the field errors namely results. This fact makes a shim procedure for adjusting an optimum base field homogeneity difficult.
The present invention is directed to the problem of reducing these errors which lead to hysteresis to a harmless amount in a homogeneous field magnet having the features indicated above.